Metal-Free Room-Temperature Phosphorescence from Amorphous Triarylborane-Based Biphenyl

In this paper, we have disclosed efficient room-temperature phosphorescence (RTP) in the amorphous state from a triarylborane-based biphenyl derivative, 4,4′-dibromo-2-dimesitylboryl-2′-(N,N-dimethylamino)­biphenyl (Br2-BN-BPh). The phosphorescence quantum yields of vigorously ground powder and drop-cast film are 0.36 and 0.22, respectively. Owing to the presence of two bulky substituents at the lateral positions, the existence of the intramolecular π–π interaction and the B···N electronic interaction, this compound is intramolecularly rigid, which is helpful to retard the vibrational motions and thus suppress the nonradiative decay in the solid state. In addition, this compound has a very small ΔES‑T due to the well separation of HOMO and LUMO as a result of the significant twisting of the biphenyl skeleton. The combination of the small ΔES‑T and heavy atom effect of the bromo substituent is expected to enhance the spin–orbital coupling and thus increase intersystem crossing. Consequently, both the steric and electronic effects of the dimesitylboryl group are important for the efficient RTP property of Br2-BN-BPh, especially in the amorphous state.